According to the International Electrotechnical Commission (IEC), external insulation is defined as “distances in atmospheric air, and the surfaces in contact with atmospheric air of solid insulation of the equipment which are subject to dielectric stresses and to the effects of atmospheric and other external conditions such as pollution, humidity, vermin, etc” [IEV 604-03-02]. This is the type of insulation dealt with in this patent application.
According to IEC Standard 71-1 (1996) dielectric stresses have several origins, the most basic of which is continuous voltages which originate from the system operation under normal operating conditions. This is the type of voltage or dielectric stress origin dealt with in this patent application.
Failure (flashover) of external insulation under normal operating voltage normally takes place when insulating surfaces are exposed to critical pollution conditions. Flashovers of insulators under normal operating voltage are characterized by several stages: flow of leakage current due to surface conductivity, formation of dry bands, bridging the dry bands by electric arcs and finally propagation of the arcs to span the whole length of the surface insulation. Sparkovers of air insulation on the other hand do not normally occur under system operating voltage since such voltages are normally too low to cause sparkover of air gaps.
Such gaps however do sparkover under the effects of lightning overvoltages caused by direct or induced lightning. The mechanism of the sparkover in this case involves positive and negative streamers emanating from the high voltage and ground terminals (electrodes). Of particular importance is the positive streamer which, due to its lower voltage gradient, is capable of spanning longer insulating distances. This type of sparkover is not preceded by the flow of any significant leakage current.
Similar sparkovers of air insulation can occur due to system overvoltages occurring due to faults and switching operations. Here air gap sparkover can occur, without flow of leakage current, by the streamer mechanism, described above. More importantly and particularly at extra-high-voltage systems, positive streamers can result in the formation of a positive leader discharge, with considerably lower voltage gradient and accordingly having the ability to span much longer insulating distances.
In this patent application we will deal with a special type of flashover/sparkover streamer/leader mechanism recently discovered and for which the name “Fast Flashover” has been coined. These Fast Flashovers have some particular characteristics:                1. Fast Flashovers occur under normal system operating voltages without any effect of lightning or switching operations.        2. Fast Flashovers occur without any significant flow of leakage current (contrary to the case of pollution flashovers).        3. The last characteristic makes Fast Flashovers particularly dangerous because of the difficulty in predicting them, particularly in cases involving personnel safety        4. Positive streamers represent a prerequisite for the occurrence of a Fast Flashover, so that inhibiting positive streamers constitutes the most logical means of eliminating Fast Flashovers.        
Combating fast flashover by either increasing the length of the insulator (gap) or by introducing insulating sheds may not always be practical or economic.
An object of the present invention is therefore to reduce the risk of such fast flashovers by inhibiting the development of streamers under different atmospheric conditions with the insulators only exposed to the system operating voltage.
At present there is no known device for reducing the risk of a streamer initiated flashover on a high voltage insulator under normal operating voltage.
US Patent publication No. 2004251700 (HESSE) discloses safety devices and methods for allegedly improving electrical safety of insulative tools. In particular, it is applied to an elongated insulative tool of a certain length with a substantially circular cross section having a cross sectional diameter and outer circumference. The device comprises a body which may be a substantially circular disc with an inner opening for the elongated insulative tool to position there through, and the inner opening has a bore diameter that is substantially the same as or greater than the diameter of the elongated insulative tool. However, test conducted on an embodiment of such device, a bare toroird, revealed that it is not effective in reducing the risk of fast flashovers or pollution flashovers.
A Wet/Dry Glow-Based Streamer Inhibitor, disclosed in U.S. provisional patent application filing No. 60/738,990, which is incorporated by reference, although not designed to affect flashovers on transmission lines, possesses many physical similarities to the invention disclosed here within but it has an entirely different application. While the purpose of a Wet/Dry Glow-Based Streamer Inhibitor (U.S. provisional patent application filing No. 60/738,990) is to reduce exposure of structures, transmission lines and substations to direct lightning strokes, the present application deals with reducing the risk of a flashover on high voltage power transmission systems under normal operating voltage. Inhibition of positive streamers is fundamental to both applications.
There is therefore a need for a device that can prevent flashovers on or across high voltage insulators conventionally used in power systems, such as streamer initiated flashovers, including streamer-initiated or fast flashovers.